1. Field of the Invention
The invention relates to a continuously working press which includes a press ram, a press table spaced apart from the press ram with an adjustable press gap being formed therebetween, drive drums and return drums, and first and second flexible, endless steel belts which are guided around the press table and the press ram via the drive drums and the return drums.
2. Discussion of the Related Art
It has been technologically proved that the best physical data, such as transverse tensile strength and bending strength for substances such as chipboards, are obtained when rapid pressure build-up occurs upon contact with the material to be pressed. That is, with the start of pressing, this material to be pressed is immediately compressed at a very high pressure up to the maximum applied pressure. This provides a very uniform and rapid heat transfer from outside to inside within the chip structure. Furthermore, in the course of the effective heat transfer under immediate action of pressure, preliminary hardening of the cover layer is no longer possible. Since preliminary hardening requires more sanding of material, these favorable technological conditions also provide the best economic preconditions by requiring less sanding of material. These requirements are intended to be met by providing continuously working presses which, in their entry area comprising the area following the entry gap predetermined by the return drums for the press belts, can set a pressure variation curve which can be adapted to the particular press tasks and operating conditions. The entry gap is normally set in a stationary position in a wedge shape with a cross-section decreasing in the entry direction, the intention being able to adjust the device to exert more or less pressure over its length on the material to be pressed as the material enters the press. In the press according to German Offenlegungsschrift 2,205,575, which is provided with a rolling-bearing chain, pressure pieces are arranged in the press gap between the rolling-bearing entrance and the return drums for the press belts. In the relevant area, these pressure pieces exert a selectively adjustable pressure on the material to be pressed. As a result, these pressure pieces set the entry gap more or less wide. In this embodiment, the steel belt is merely returned in the front area. This is then followed by a virtually pressureless sliding section and only then by the actual rolling-bearing entrance, where the pressure is gradually raised from 0 up to the maximum applied pressure.
Of disadvantage here is the fact that, after the first contact of the material to be pressed under pressure action by the return drums and the pressure bodies, the pressure is relieved twice, so that there is the risk of the initially hardened and embrittled cover layer being damaged by transverse cracks due to the slightest expansion (breathing) of the chip mat and thus of the overall strength of the finished chipboard being reduced.
The fact that the roller bars, although inserted orthogonally in the entry area, lose their predetermined synchronous running with identical spacing between one another in the compression build-up area due to defective chip fillings, given as an example in chipboard manufacture, can be cited as a further disadvantage. This can lead to individual roller bars running up against one another and thus to their destruction.